Apparatus for detecting small rotations

ABSTRACT

Slight rotations are detected by the variation, according to the angle of incidence, of dephasing introduced by a total reflection between the light polarized perpendicularly to the plane of incidence, and that which is polarized parallel to this plane. The apparatus includes a polarized light source, a prism with a parallelogram-shaped section in which the light undergoes four total reflections, a dephasing plate, an analysis polarizer, and a light detector.

a, 2/ z/7 O Muted State [151 3,635,563 Mouchart Jan, E8, 3972 541APPARATUS FOR DETECTING SMALL 3 32%: 3132; gean. Jr. et a1.

yce ROTATIONS 3,474,255 10/1969 White ..250/225 [72] Inventor: JacquesMouchart, LHay-Les-Roses,

France FOREIGN PATENTS OR APPLICATIONS 73 Assignee: Compagnie GeneraleD'Electricite Paris, 1,033,889 1953 France ..356/1 14 France PrimaryExaminerRonald L. Wibert Flledl 1969 Assistant ExaminerJ. Rotherberg[21] APPL NW 877,724 Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak[57] ABSTRACT F t D ta [30] orelgn Applicanon Pnon y a Slight rotationsare detected by the variation, according to the NOV 18, 1968 France..l74,26l angle of incidence, of dephasing introduced by a totalreflection between the light polarized perpendicularly to the plane Clof incidence, and that which is polarized parallel to this plane. 2356/1 18 The apparatus includes a polarized light source, a prism with a[51] Int. Cl. ..G0ln 21/40 arallelogram-shaped section in which thelight undergoes [58] Field of Search ..250/225; 350/152, 285; four totalreflections, a dephasing plate, an analysis polarizer,

356/1 14-1 18 and a light detector.

6] I References Cited 4 Claims, 1 Drawing Figure UNITED STATES PATENTS2,666,355 1/1954 Trumit ..356/l16 APPARATUS FOR DETECTING SMALLROTATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention concerns the detection of small rotations, such as ofa support, by study of a light beam emerging from an optical systemfixed to the support; the light beam, which reaches the optical systemand gives rise to this emergent beam, being fixed. The invention isequally applicable to the detection of small rotations of the beamarriving at the fixed optical system.

2. Description of the Prior Art Various apparatus for detecting smallrotations of a support with the aid of an optical system fixed to thesupport are known. For example, a fixed light beam may be sent to amirror fixed to the support, and the illumination may be measured atsome point which the reflected beam more or less reaches, depending uponthe angular position of the mirror. Such apparatus, although of highsensitivity, are generally cumber some.

SUMMARY OF THE INVENTION The present invention obviates thesedisadvantages and relates to an apparatus for detecting small rotationsof a transparent block in relation to the direction of propagation of anincident polarized light beam, comprising at least one reflecting face,from which the incident light beam is internally reflected at an angleof incidence in the neighborhood of the critical angle of totalreflection. It comprises, in addition, means for detecting the lightemerging from the block, characterized in that the detecting means isdisposed in the path of the beam reflected from the reflecting face. Ananalyzing polarizer is disposed in this path between the face and thedetecting means.

BRIEF DESCRIPTION OF OF THE DRAWING The single FIGURE is a sectionalview of the apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT It is known that when a lightray is internally reflected to a material having a relatively highoptical index of refraction (glass) from a surface separating thismaterial from another material having a relatively low index (air) at anangle in the neighborhood of the critical angle of total reflection, thereflection introduces between the components of the beam, polarized inparallel relationship and perpendicularly to the plane of incidence, aphase difference which varies very rapidly as a function of the angle ofincidence. The phase difference angle a is defined as a function of theincidence i and of the relative index n of the external medium by theequation:

Since a linearly polarized light beam is converted by the reflectioninto an elliptically polarized beam this has the result that theeccentricity of the polarization ellipse will vary rapidly with theangle of incidence. Now, it is easy to measure the eccentricity of thepolarization ellipse by measuring the lu minous intensity passingthrough an appropriately oriented analyzing polarizer.

There will be seen in the FIGURE a monochromatic light beam 1 polarizedat 45 to the plane of the FIGURE. This beam penetrates the interior of aprismatic block 2, of which the cross section is shown in the form of aparallelogram in the FIGURE. This block consists of a glass having ahigh optical index of refraction, which may be, for example, the glass F4820 of Societe Francaise SOVIREL having an index of refraction of L9.This light beam is reflected four times in succession, alternately fromtwo plane and parallel reflection faces of the block 2. Thesereflections advantageously take place at angles close to the criticalangle of total reflection,

tan

but the reflection advantageously remains total regardless of thevariation of the angle of incidence, at least as long as this variationremains within practical limits.

By multiplication of the'number of reflections it is possible tomultiply the phase difference between the luminous vibrations, paralleland perpendicular to the plane of incidence, with is the same for allthe reflections. The number of such reflections is advantageously aneven number, so that the emergent beam 3 is parallel to the incidentbeam 1. The polarization of this beam 3 is elliptical, and the variationof the eccentricity of the ellipse, as a function of the angle ofincidence, is very rapid. The eccentricity is detected with the aid ofan analyzing polarizer 5, the plane of polarization of which isappropriately oriented, and with the aid of a light detector 6. Thesensitivity may be improved by interposing a phase-shifting plate 4between the block 2 and the polarizer 5, so as to obtain, for example,linearly polarized light when the angle of incidence is at its normalvalue. The analyzing polarizer 5 may then be so oriented as to allow thepassage of only the light polarized perpendicularly to the plane of thislinear polarization and then receives light only when the angle ofincidence departs from its normal value.

The beam 1 may be produced, for example, by a fixed laser 7, the block 2being fixed to a support, the small rotations of which must be detected.The elements 4, 5 and 6 may be either fixed or mounted on the support.

The overall dimensions of such an apparatus may be made small, thereduction of the length of the path of the light beam outside the block2 having no influence on the sensitivity. On the other hand, theemergent beam always remains parallel to the incident beam, whereby thedifficulty of assembly is reduced.

What is claimed is:

1. Apparatus for detecting small rotations of an element made oftransparent material comprising: a luminous source for directing a fixedpolarized luminous beam into said element, said element comprising twoplane and parallel reflecting faces on which the beam is alternatelyreflected inside said element and wherein at least one reflectingsurface portion of said element in the-path of the luminous beam has theangle of incidence of said beam on said reflecting portion in theneighborhood of the critical angle of total reflectance thereof, ananalyzing polarizer disposed in the path of the luminous beam emergingfrom said element and means for detecting the intensity of the lightemerging from said polarizer, whereby; the variation of intensity ofsaid beam is representative of the rotation of the said element aroundan axis other than an axis perpendicular to said reflecting portion andthe axis of propagation of said fixed polarized luminous beam.

2. The apparatus according to claim 1, wherein the form of said elementis such that the beam internally undergoes the same number ofreflections from the two plane and parallel reflecting faces.

3. Apparatus for detecting small rotations of an element made oftransparent material comprising: a luminous source for directing a fixedpolarized luminous beam into said element, said element comprising inletand outlet faces respectively through which the light beam enters andleaves which are plane and perpendicular to the light beam and whereinat least one reflecting surface portion of said element lies in the pathof the luminous beam in said element with the angle of incidence of saidbeam on said reflecting portion being in the neighborhood of thecritical angle of total reflection thereof, an analyzing polarizerdisposed in the path of said luminous beam emerging from said elementand means for detecting the intensity of light emerging from saidpolarizer, whereby; the variation of intensity of said beam isrepresentative of the rotation of said element around an axis other thanan axis perpendicular to said reflecting portion and the axis ofpropagation of said fixed polarized luminous beam.

4. The apparatus according to claim 3, wherein said transparent elementhas the form of a prism whose cross section is a parallelogram.

1. Apparatus for detecting small rotations of an element made oftransparent material comprising: a luminous source for directing a fixedpolarized luminous beam into said element, said element comprising twoplane and parallel reflecting faces on which the beam is alternatelyreflected inside said element and wherein at least one reflectingsurface portion of said element in the path of the luminous beam has theangle of incidence of said beam on said reflecting portion in theneighborhood of the critical angle of total reflectance thereof, ananalyzing polarizer disposed in the path of the luminous beam emergingfrom said element and means for detecting the intensity of the lightemerging from said polarizer, whereby; the variation of intensity ofsaid beam is representative of the rotation of the said element aroundan axis other than an axis perpendicular to said reflecting portion andthe axis of propagation of said fixed polarized luminous beam.
 2. Theapparatus according to claim 1, wherein the form of said element is suchthat the beam internally undergoes the same number of reflections fromthe two plane and parallel reflecting faces.
 3. Apparatus for detectingsmall rotations of an element made of transparent material comprising: aluminous source for directing a fixed polarized luminous beam into saidelement, said element comprising inlet and outleT faces respectivelythrough which the light beam enters and leaves which are plane andperpendicular to the light beam and wherein at least one reflectingsurface portion of said element lies in the path of the luminous beam insaid element with the angle of incidence of said beam on said reflectingportion being in the neighborhood of the critical angle of totalreflection thereof, an analyzing polarizer disposed in the path of saidluminous beam emerging from said element and means for detecting theintensity of light emerging from said polarizer, whereby; the variationof intensity of said beam is representative of the rotation of saidelement around an axis other than an axis perpendicular to saidreflecting portion and the axis of propagation of said fixed polarizedluminous beam.
 4. The apparatus according to claim 3, wherein saidtransparent element has the form of a prism whose cross section is aparallelogram.